You've always had siblings, you've mentioned the siblings to your class a few times, it wasn't until you rushed in through the dorms sibling in tow that the others couldn't help but feel curious, Kirishima especially, especially considering the fact Kirishima himself had literally no siblings, so it always made him a little curious to see sibling dynamics, after only basing his thoughts off of what he's seen in media, he could always tell media was cheesy and lying about siblings, so when he saw you rush through and go into your room with siblings in tow, he decided to knock on the door

"Uhh, hey, you in there bro?" Kirishima says, knowing damn well you were in there just wanting to see you